1. Field of the Invention
The present invention relates to a method of image processing by correcting read-out image data in an image reading apparatus such as an image scanner, copying machine or multifunction peripheral.
2. Description of the Related Art
When a saddle-stitch bound document such as a book or magazine is read or copied, the document surface is opened facedown and placed on a transparent document platen of an image scanner or copying machine and an image is read from below the document platen.
When reading an image in this orientation, the binding portion which is the section in proximity to the right and left borders of the opened pages of the document or the end (hereafter “front edge”) of the stacked page leaves on both sides of the document surface may be separated from the platen. As a result, a shadow is produced on the binding portion or the front edge of the read-out image data and has an adverse effect on the quality of the output image. In particular, the image is darkened in proximity to the binding portion and when the darkened range includes letters, those letters are difficult to make out or read. This tendency becomes more pronounced when using thick documents.
when image data having the above features are printed, the problem of unnecessary consumption of toner or ink arises.
When the reading operation is performed to avoid the above problems by placing the binding portion of the copied document in close contact with the platen by applying pressure to prevent floating from the platen, damage to the copied document may result.
In this context, U.S. Pat. No. 6,330,050 discloses a method using imaging processing to correct a shadow in which, for example, an image correction apparatus estimates a three-dimensional shape of a book based on outer contour lines of the page in the read image.
U.S. Pat. No. 7,072,527 discloses a method of correcting a difference in the floating height from the platen between the top and bottom of the document binding portion resulting from the manner in which the document on the platen is pressured.
Japanese Patent Application Laid-Open No. 2005-115768 discloses a method of specifying the hue in background portions of the document in the document image data and applying a luminance correction to pixels which display a hue similar to the specified hue and are regarded as the background pixels.
U.S. Patent Application Publication No. 2008/0024845 discloses a method of suitable correction of luminance unevenness resulting from the shadow even when a document includes figures or photographs in background portions of the document or is printed in color. In this case, the figures or photographs are not modified for processing.
However, book documents, and in particular pages including high-quality photographs, may use paper with glossy characteristics (hereinafter referred to as “glossy paper”). Examples of glossy paper include catalogues or photographic magazines.
FIG. 24A is a sectional view seen in a direction from the top to the bottom of a book document 2401 which is opened and placed onto a document platen 2402.
The document surface has a planar portion 2410 and a curved portion 2411. The planar portion 2410 denotes a range other than the binding portion of the book document 2410 and is in close contact with the document platen 2402. The curved portion 2411 denotes the document surface going from the planar portion 2410 towards the binding portion of the right and left pages. The document surface is floating from the document platen 2402 and forms a curved face.
Since illumination light is weakened about the center of the floating range of the curved portion 2411, a shadow is produced in the binding portion.
An image reading apparatus reads a reflected document. Illumination light 2402 from a light source 2403 is irregularly reflected on the document surface and this diffused light 2406 becomes incident onto an image sensor 2405 such as a CCD sensor which then performs photoelectric conversion. Thus, an image reading apparatus acquires digital image data of the document. This configuration functions effectively for the planar portion 2410 where the document is in close contact with the document platen.
In the arrangement shown in FIG. 24A, since specular reflection light 2407, which is illumination light undergoing specular reflection on the document surface, is reflected in the direction of the solid arrow in FIG. 24A, the image sensor only receives diffuse light which corresponds to the document surface density.
However, since the document surface is curved near the binding portion of the book document 2401, the specular reflection light at the reading positions shown in FIG. 24B propagates more towards the image sensor 2405 than the light shown in FIG. 24A. The roughness of the document surface also has some effect on the signal output of the image sensor. In particular, the direction of the incident angle of illumination light and the direction of incident light on the image sensor are symmetrical at the reading position shown in FIG. 24C with respect to the perpendicular direction V of the document surface and illumination light undergoing specular reflection from the document surface becomes incident on the image sensor.
As described above, when a document surface is floating with a curvature, and in particular, when the document is glossy paper, a large amount of illumination light undergoes specular reflection, which is incident upon the image sensor, depending on the reading position. As a consequence, a brightness of the illumination light is higher than the document density and it becomes difficult to acquire image data of the original document.
In this context, Japanese Patent Application Laid-Open No. 2007-166431 discloses a technique of using image processing to correct a shadow on a binding portion thereby correcting luminance abnormalities resulting from specular reflection from glossy paper. This image processing apparatus models the effect of specular reflection light predicted from the floating of the document surface in the binding portion and the curvature of that surface. And a specular reflection component is corrected by subtracting the specular reflection component resulting from the curvature of the document surface, from the reading image data. More precisely, the processing apparatus uses a ratio fixed by the model to extend a specular reflection range predicted in the neighborhood, based on a width of the dark binding portion that does not produce specular reflection.
However, the characteristics of surface reflection from glossy paper are various and many documents do not use glossy paper. Therefore, a uniform treatment of the effect of specular reflection is not determined. Furthermore, there are clearly large differences in the curvature of the document surface at the binding portion, which depend on the thickness or binding way of the bound document, whether the open page is near the cover page of the bound document or whether it is in the middle of the total number of pages. Therefore an undesirable correction may be caused if a range for specular reflection correction is estimated from only the range of the shadow in the binding portion.
Further, in the above method, a low luminance range exceeding a predetermined threshold when compared to the document background luminance of a planar portion, is considered to be the range of the shadow in the binding portion. However, this method is limited to documents in which the background of the document is a uniform white or pale density. For example, since the background of a document surface containing a plurality of photographs may not have a uniform white or pale density, in such a case, application of this method is not sufficiently effective. This is a practical obstacle because photographs are often printed in glossy pages that tend to produce specular reflection.
In other words, current techniques may result in performing undesirable corrections depending on contents of the bound document, the type of paper or the method of placement.